The Sapphire Ring — C. Dale Brittain

Antonia is hoping for a quiet semester at the wizards' school when her life is upended. She is kidnapped by women practitioners of darker spells, who want her to fulfill the prophecy of the Starlight Raven to overcome their enemies. Even if she escapes from them, another young woman who tries to help her will still be in their hands. Meanwhile hidden plots threaten the wizards' school itself. And then there is the sapphire ring from the Land of Wild Magic, long ago lost, but which may convey a dangerous power for those who wear it.